CLI: Add multi-session signing support (#8927)

* SDK: Add `NullSigner` implementation

* SDK: Split `Transaction::verify()` to gain access to results

* CLI: Minor refactor of --sign_only result parsing

* CLI: Enable paritial signing

Signers specified by pubkey, but without a matching --signer arg
supplied fall back to a `NullSigner` when --sign-only is in effect.
This allows their pubkey to be used for TX construction as usual,
but leaves their `sign_message()` a NOP. As such, with --sign-only
in effect, signing and verification must be done separately, with
the latter's per-signature results considered

* CLI: Surface/report missing/bad signers to user

* CLI: Suppress --sign-only JSON output

* nits

* Docs for multi-session offline signing

docs/src/offline-signing/README.md

@@ -82,6 +82,72 @@ Output
 4vC38p4bz7XyiXrk6HtaooUqwxTWKocf45cstASGtmrD398biNJnmTcUCVEojE7wVQvgdYbjHJqRFZPpzfCQpmUN
 ```
 
+## Offline Signing Over Multiple Sessions
+
+Offline signing can also take place over multiple sessions. In this scenario,
+pass the absent signer's public key for each role. All pubkeys that were specified,
+but no signature was generated for will be listed as absent in the offline signing
+output
+
+### Example: Transfer with Two Offline Signing Sessions
+
+Command (Offline Session #1)
+
+```text
+solana@offline1$ solana transfer Fdri24WUGtrCXZ55nXiewAj6RM18hRHPGAjZk3o6vBut 10 \
+    --blockhash 7ALDjLv56a8f6sH6upAZALQKkXyjAwwENH9GomyM8Dbc \
+    --sign-only \
+    --keypair fee_payer.json \
+    --from 674RgFMgdqdRoVtMqSBg7mHFbrrNm1h1r721H1ZMquHL
+```
+
+Output (Offline Session #1)
+
+```text
+Blockhash: 7ALDjLv56a8f6sH6upAZALQKkXyjAwwENH9GomyM8Dbc
+Signers (Pubkey=Signature):
+  3bo5YiRagwmRikuH6H1d2gkKef5nFZXE3gJeoHxJbPjy=ohGKvpRC46jAduwU9NW8tP91JkCT5r8Mo67Ysnid4zc76tiiV1Ho6jv3BKFSbBcr2NcPPCarmfTLSkTHsJCtdYi
+Absent Signers (Pubkey):
+  674RgFMgdqdRoVtMqSBg7mHFbrrNm1h1r721H1ZMquHL
+```
+
+Command (Offline Session #2)
+
+```text
+solana@offline2$ solana transfer Fdri24WUGtrCXZ55nXiewAj6RM18hRHPGAjZk3o6vBut 10 \
+    --blockhash 7ALDjLv56a8f6sH6upAZALQKkXyjAwwENH9GomyM8Dbc \
+    --sign-only \
+    --keypair from.json \
+    --fee-payer 3bo5YiRagwmRikuH6H1d2gkKef5nFZXE3gJeoHxJbPjy
+```
+
+Output (Offline Session #2)
+
+```text
+Blockhash: 7ALDjLv56a8f6sH6upAZALQKkXyjAwwENH9GomyM8Dbc
+Signers (Pubkey=Signature):
+  674RgFMgdqdRoVtMqSBg7mHFbrrNm1h1r721H1ZMquHL=3vJtnba4dKQmEAieAekC1rJnPUndBcpvqRPRMoPWqhLEMCty2SdUxt2yvC1wQW6wVUa5putZMt6kdwCaTv8gk7sQ
+Absent Signers (Pubkey):
+  3bo5YiRagwmRikuH6H1d2gkKef5nFZXE3gJeoHxJbPjy
+```
+
+Command (Online Submission)
+
+```text
+solana@online$ solana transfer Fdri24WUGtrCXZ55nXiewAj6RM18hRHPGAjZk3o6vBut 10 \
+    --blockhash 7ALDjLv56a8f6sH6upAZALQKkXyjAwwENH9GomyM8Dbc \
+    --from 674RgFMgdqdRoVtMqSBg7mHFbrrNm1h1r721H1ZMquHL \
+    --signer 674RgFMgdqdRoVtMqSBg7mHFbrrNm1h1r721H1ZMquHL=3vJtnba4dKQmEAieAekC1rJnPUndBcpvqRPRMoPWqhLEMCty2SdUxt2yvC1wQW6wVUa5putZMt6kdwCaTv8gk7sQ \
+    --fee-payer 3bo5YiRagwmRikuH6H1d2gkKef5nFZXE3gJeoHxJbPjy \
+    --signer 3bo5YiRagwmRikuH6H1d2gkKef5nFZXE3gJeoHxJbPjy=ohGKvpRC46jAduwU9NW8tP91JkCT5r8Mo67Ysnid4zc76tiiV1Ho6jv3BKFSbBcr2NcPPCarmfTLSkTHsJCtdYi
+```
+
+Output (Online Submission)
+
+```text
+ohGKvpRC46jAduwU9NW8tP91JkCT5r8Mo67Ysnid4zc76tiiV1Ho6jv3BKFSbBcr2NcPPCarmfTLSkTHsJCtdYi
+```
+
 ## Buying More Time to Sign
 
 Typically a Solana transaction must be signed and accepted by the network within